1,721,035 research outputs found
Testing the hypothesis of orbital (eccentricity) influence on Earth's magnetic field
No full textWe test a recent suggestion that orbital eccentricity has influenced the geomagnetic field by performing a range of spectral analyses, including estimation of statistical confidence limits, on a 2.2-million-year paleomagnetic inclination record from the western Caroline Basin (WCB). Our analyses indicate that the claimed ~100-kyr signal, while present over discrete portions of the WCB paleomagnetic record, is not statistically significant for the entire record and is not modulated by the 404-kyr eccentricity component as would be expected if the signal was influenced by orbital eccentricity. This signal also has highly variable phase compared to orbital eccentricity variations and the paleomagnetic record shows no statistically significant coherency with the orbital signal. Our results from a nearby coeval western Philippine Sea record support the present analysis. Together, these data suggest that the hypothesis of orbital energization of the geomagnetic field has yet to be convincingly demonstrated
Three million years of monsoon variability over the northern Sahara
No full textWe present a 3 million year record of aeolian dust supply into the eastern Mediterranean Sea, based on hematite contents derived from magnetic properties of sediments from Ocean Drilling Program Site 967. Our record has an average temporal resolution of ?400 years. Geochemical data validate this record of hematite content as a proxy for the supply of aeolian dust from the Sahara. We deduce that the aeolian hematite in eastern Mediterranean sediments derives from the eastern Algerian, Libyan, and western Egyptian lowlands located north of the central Saharan watershed (?21°N). In corroboration of earlier work, we relate dust flux minima to penetration of the African summer monsoon front to the north of the central Saharan watershed. This would have enhanced soil humidity and vegetation cover in the source regions, in agreement with results from "green Sahara" climate models. Our results indicate that this northward monsoon penetration recurred during insolation maxima throughout the last 3 million years. As would be expected, this orbital precession-scale mechanism is modulated on both short (?100-kyr) and long (?400-kyr) eccentricity time scales. We also observe a strong expression of the ?41-kyr (obliquity) cycle, which we discuss in terms of high- and low-latitude mechanisms that involve Southern Hemisphere meridional temperature contrasts and shifts in the latitudes of the tropics, respectively. We also observe a marked increase in sub-Milankovitch variability around the mid-Pleistocene transition (?0.95 Ma), which suggests a link between millennial-scale climate variability, including monsoon dynamics, and the size of northern hemisphere ice sheets
A new model for a magnetoreceptor in homing pigeons based on interacting clusters of superparamagnetic magnetite
No full textWe present a new model of magnetic-field reception in magnetite-containing nerve terminals, which have recently been identified in the upper-beak skin of homing pigeons. The potentially magnetoreceptive nerve cells comprise chain-like aggregates with up to 20 closely spaced clusters of superparamagnetic (SP) magnetite. We designed experiments on superparamagnetic model systems to simulate the behaviour of the aggregates in varying magnetic fields. Magnetic-field induced interactions between the clusters in an aggregate gives rise to attractive and repulsive forces between the clusters. The resulting stress on the surrounding cellular structures varies with field direction and intensity. Our model is able to explain the principal features of the magnetic sense in homing pigeons as derived from behavioural experiments
A 3-Myr mineral magnetic record of Saharan dust in input into the eastern Mediterranean: linking magnetic data with climate variability over northern Africa (abstract of paper presented at AGU Fall Meeting, San Francisco, 8-12 Dec 2003)
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Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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